Add background color style to output svg's

[jg123]

📑 Summary

Output svg's have a transparent background which doesn't look good against github's dark mode themed pages. Set the background-color in the style tag of the svg to set the background color to white (default) or whatever a user chooses with the -b option.

Resolves some user requests in this ticket: mermaid-js/mermaid#1553

📏 Design Decisions

Shouldn't crash. Tests pass.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added unit/e2e tests (if appropriate)
• 🔖 targeted master branch

[MindaugasLaganeckas]

@jg123 I am a bit sick at the moment. I will get back to you this or next week. Thank you for your contribution.

[jg123]

Hope you feel better soon!

[MindaugasLaganeckas]

Thanks for the PR! 😄

[linustannnn]

Hi, can you provide an example as to how to adjust the bg color? I'm not sure what you mean by setting the style tag. Is there a way to change it in here:

%%{
  init: {
    'theme': 'base',
    'themeVariables': {
      'primaryColor': '#fff',
      'primaryTextColor': '#000',
      'primaryBorderColor': '#000',
      'lineColor': '#000',
      'secondaryColor': '#fff',
      'secondaryTextColor': '#000',
      'secondaryBorderColor': '#000',
      'tertiaryColor': '#fff',
      'tertiaryTextColor': '#000',
      'tertiaryBorderColor': '#000'
    }}
}%%